Maynard, MA, USA: Beacon-Villager newspaper column on local history, observations on nature and recreational activities, plus an occasional health-related article. Columns from 2009-11 collected into book "MAYNARD: History and Life Outdoors." Columns from 2012-14 collected into book "Hidden History of Maynard." - David A. Mark

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Tuesday, February 2, 2010

Avoid Overhydration and Hyponatremia

Also see May 2013: "Water - How Much to Drink?"This article is about the perils of overhydration - rarer than dehydration - but with its own adverse consequences. DO plan to drink during any endurance event such as a 100 mile (century) bicycle ride. Under warm & dry conditions an athlete can lose a quart (two pounds) of water an hour. According to a study by T.D. Noakes, of 2,135 athletes completing endurance events found that 50% were dehydrated at the finish, defined in the study as having lost more than 3% of pre-event body weight. [Proc Natl Acad Sci 2005;102:18550-55].

Weight losses of up to 2% are not expected to affect performance. Long training rides can be used to learn how much to drink to keep weight loss in the 0% to 2% range. There does not appear to be any advantage to pre-loading fluids before the start, and not doing so may save you time lost waiting in line at the Port-O-Potties at the first rest stop of a century. Be mindful that lack of urine production is not necessarily a sure sign of dehydration (see below).

DON'T overhydrate before, during, or after the ride. The popular dogma is “Drink before you are thirsty.” The dogma is wrong and potentially dangerous. With today’s cult of hydration, 5% to 20% of people completing endurance events will weigh more at the end than when they started! Studies have shown that when athletes drink to satisfy thirst they tend to replace only about 75% of the water they are losing during the endurance event. This, rather than drinking to not lose any weight, appears to be the safer target. Full recovery from the mild, event-related dehydration will take place after the event is over. Overhydrating, measurable as gaining weight during the course of the event, does not improve performance.

The health risk from overhydration is that it lowers sodium concentration in the blood. The scientific term for this is hyponatremia. In that study mentioned above, 10% of the athletes had gained weight by the end of the event, 3% had blood sodium below the normal range, and 1% had symptoms associated with low sodium. While puffy hands and ankles are not a serious health problem, swelling of the brain is! The problem here is that the brain is encased in a rigid container – the skull – and cannot expand.

From mild to severe, the signs and symptoms of overhydration can include one or more of the following: headache, nausea, puffiness (face, ankles, hands), dizziness, confusion, agitation, vomiting, respiratory distress, stupor, coma, seizures, and (very, very rarely) death.

Sports drinks do not contain enough sodium to prevent hyponatremia.

Contributing to the incidence of overhydration is that fact that for some people, prolonged exercise turns off urine production even if they are overhydrated. The effect can continue for hours after exercise stopped. Thinking that “No pee” means dehydration, these people continue drinking water and sports drinks in an attempt to trigger urine production. The best protection is to check post-event weight against pre-event weight, and not drink anything more than small amounts of fluids if you are heavier afterwards compared to before.

The International Marathon Medical Directors Association recommends athletes drink to satisfy thirst. The IMMDA also recommends that for marathon participants any post-event weight gain constitutes justification for a medical consultation. [Clin J Sport Med 2006;16:283-92]. The same thinking would apply to bicyclists.

1 comment:

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